JPS62875B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing a heat-resistant, high-strength sintered body, particularly a silicon nitride (Si ₃ N ₄ )-based sintered body. Silicon nitride sintered bodies (ceramics), which are mainly composed of Si ₃ N ₄ , have excellent high-temperature strength, thermal shock resistance, and corrosion resistance, so they are attracting a lot of attention as heat-resistant structural materials, such as gas turbine parts and heat exchanger materials. is being sent to us. However, _{Si3N4 powder} is
Since sinterability is extremely poor by itself, sintering aids such as MgO, Al ₂ O ₃ , Y ₂ O ₃ , SiO ₂ and oxides of Group A elements of the periodic table are usually added to sinter. will be carried out. However, the physical properties of the final sintered body obtained depend on the properties of the so-called grain boundary phase formed between Si ₃ N ₄ particles due to the reaction between Si ₃ N ₄ (containing incidental impurities) and these sintering aids. is largely dominated by For example, when Y ₂ O ₃ and Al ₂ O ₃ are used as sintering aids, Si 3 N 4 and Si ₃ N ₄ are added to the grain boundary phase by sintering.
A phase of a mixture of Y ₂ O ₃ based compounds, Si-Al-Y-N-O based glass, etc. is formed. Of this mixture, Si-Al-Y-N-O glassy, Si-Al-Y-
It is known that O-based glass deteriorates the physical properties of a sintered body. That is, in order to obtain a high-quality sintered body, the grain boundaries must be occupied by a stable phase consisting of a Si ₃ N ₄ /Y ₂ O ₃ type structure, and the sintering process must be carried out so as to minimize the formation of the above-mentioned glassy substance. Conditions need to be controlled.
Particularly in the case of large, thick-walled bodies that have industrial utility, vitreous remains, making it difficult for the silica components (SiO, SiO ₂ ) inside the sintered body to dissipate, making it unavoidable that the structure will become non-uniform. By the way, as a measure to improve this, high-quality Si ₃ N ₄ Y ₂ O ₃ , which is a grain boundary phase material, is synthesized in advance and α
A homogeneous, dense, high-strength, heat-resistant material can be produced by hot pressing a Si ₃ N ₄ -Si ₃ N ₄ .Y ₂ O ₃ type mixed powder as a raw material. However, this type of sintered body has the following problems in achieving even higher strength.
That is, Si ₃ N ₄ powder as a raw material usually contains silica components (SiO, SiO ₂ ), which are inevitable and difficult to remove, as incidental impurities, in addition to aluminum oxide, iron oxide, etc. There is. Silica components generally exist on the surface of Si ₃ N ₄ particles, and Y _{2 O 3 and Y 2 O 3} are added to the sintering aids added to Si 3 N ₄ as described above.
When using Al ₂ O ₃ , Si ₃ N ₄ and Y ₂ O ₃ ,
It reacts with Al ₂ O ₃ to form a Si-Al-Y-N-O glass, and then as the silica content evaporates from the glass, Si ₃ N ₄・Y forms at the grain boundaries of the sintered body. Create a ₂ O ₃ -based high melting point compound. The series of reaction mechanisms is more difficult inside the sintered body, causing the structure to become non-uniform, but in order to avoid this, as described above,
It is a grain boundary phase material synthesized in advance in Si ₃ N ₄ .
Sintering with the addition of Si ₃ N ₄ · Y ₂ O ₃ prevents the structure from becoming non-uniform, but since there is no reaction mechanism to dissipate the silica component, the sintering with the added Si ₃ N ₄ · Y ₂ O ₃ is difficult. This leads to reactions, forming low melting point compounds such as Y ₅ (SiO ₄ ) ₃ N (H phase), and furthermore, regressing to a glassy state, which naturally limits the ability to achieve high strength. As a result of the inventors' studies to address these inconveniences, we found that in the manufacturing method for obtaining a sintered body by adding Si ₃ N ₄ Y ₂ O ₃ powder, a trace amount of aluminum nitride is added. It has been found that by doing so, the impurity silica component can be eliminated by the SiO ₂ +AlN→"Al--Si--O--N" compound forming reaction, and a more excellent heat-resistant and high-strength sintered body can be obtained. Based on such knowledge, the present invention aims to provide a method for easily producing high-quality silicon nitride sintered bodies (ceramics) containing Si ₃ N ₄ as a main component and suitable for use as heat-resistant structural materials. It is. To explain the present invention in detail below, the present invention is a silicon nitride powder containing at least 60% α-type structure.
75% by weight, general formula, (Si _3-x Al _x )N _4-x O _x・Re ₂ O ₃ (In the formula, Re is an element in group a of the periodic table, x is 0 to 0.5
A composition (formulation) containing 1 to 20% by weight of a powdered substance indicated by the number of 0.1 to 5% by weight of aluminum nitride powder under a pressure of at least 100 Kg/cm ³
This is a method for producing a high-strength heat-resistant material, which is characterized by pressure sintering at 1700-1800°C. Note that (Si _3-x Al _x )N _4-x O _x・
The majority of the Re ₂ O ₃ material, preferably 85% or more, has a structure of the Si ₃ N ₄ .Y ₂ O ₃ type (Si _3-x Al _x )N _4-x
It consists of O _x・Re ₂ O ₃ , and others include, for example, “Si—
It may contain compounds other than those mentioned above, such as Al--Re--O--N. However, the method of the present invention is carried out, for example, as follows. First, the particles are finely pulverized to a particle size of 2.5 μm or less, preferably 1.2 μm or less. Si ₃ N ₄ powder (containing incidental impurities) containing at least 60% and preferably 85% or more of Si ₃ N ₄ with an α-type structure.
Prepared as Si ₃ N ₄ raw material. On the other hand, prepare an oxide Y ₂ O ₃ of a group A element of the periodic table, such as yttrium (Y), and mix it with Si ₃ N ₄ and 1
When mixed at a molar ratio of 1 to 1 and calcined in a nitrogen atmosphere or an inert atmosphere, a powder having a Si ₃ N ₄ .Y ₂ O ₃ crystal structure and a particle size of 2.5 μm or less can be obtained. This Si ₃ N ₄・Y ₂ O ₃ may be used as is, but it may also be one in which part of the yttrium (Y) component is replaced with aluminum (Al).
The amount of Al substitution is up to 50%. In the present invention, aluminum nitride (AlN) powder is prepared as another additional component. Aluminum nitride powder is easily obtained by nitriding aluminum (Al) powder, for example.
Further, commercially available powders may be used, and fine powders having a particle size of 3 μm or less, preferably 1.5 μm or less are preferable. Next, contain 60% or more of the α-type structure prepared above.
_Si3N4 powder 98.9-75% by weight, (Si3 _{- x Alx} )N4 _-xO
x・Re ₂ O ₃ system 1-20% by weight and AlN powder 0.1-5
The weight percentages are blended or mixed, and the mixed powder is molded to give a mass of at least 100 kg/kg in a non-oxidizing atmosphere.
Pressure sintering is performed at 1700-1850 °C under a pressure of ^cm3 . For this pressure sintering, so-called hot press, high pressure gas pressure hot press, high temperature isostatic press method, etc. are appropriately selected. By doing so, a heat-resistant structural material having high strength at high temperatures can be easily produced with good reproducibility. Silicon nitride powder (Si ₃ N ₄ ), which is the main starting material in the present invention, must contain at least 60% of α-type structure. The reason for this is that if the content of α-type structure is less than 60%, a high strength, heat-resistant sintered body cannot be obtained because the proportion of long columnar crystal grains that make up the final sintered body is low. . On the other hand, (Si _3-x Al _x ) is added to the Si ₃ N ₄
The reason why the amount of Re substitution by Al in the N _4-x O _x ·Re ₂ O ₃ component is limited to at most 50% is as follows. That is, when the amount of Al substitution exceeds 50%, it is difficult to form (Si _3-x Al _x )N _4-x ·Re ₂ O ₃ crystals. However, the composition ratio of (Si _3-x Al _x ) N _4-x O _x ·Re ₂ O ³ is selected in the range of 1 to 20% by weight because the densification effect is weak at less than 1%; This is because if it exceeds 20%, the Si ₃ N ₄ powder, which is the main raw material, will be relatively reduced and the properties of the sintered body will no longer be improved. Furthermore, the reason why aluminum nitride (AlN) added to the Si ₃ N ₄ is limited to 0.1 to 5% by weight is as follows. In order to absorb the silica component that is always present in the Si ₃ CN ₄ powder as a raw material and convert it into a compound, its action and effect are not fully exhibited at less than 0.1%, and as a result, when it exceeds 5%. This is because the purity of the sintered body decreases, resulting in relatively poor density and stability, making it impossible to obtain a high-quality sintered body. Furthermore, in the present invention, a pressure of at least 100 kg/cm ³ is applied during sintering, and the sintering temperature is set to 1700 kg/cm 3 .
It is necessary to choose ~1850℃. The reason is 100Kg/
This is because unless pressure sintering is performed within a temperature range of 1700 to 1850° C. while applying a pressure of cm ³ or more, it is difficult to obtain a sintered body with the desired high temperature and high strength. As mentioned above, the starting material is a mixed powder of α-Si ₃ N ₄ powder, a small amount of (Si _3-x Al _x )N _4-x O _x・Re ₂ O ₃ type powder, and a small amount of aluminum nitride powder. According to the method of the invention, it is possible to easily produce a high-strength, heat-resistant sintered body that reliably exhibits a bending strength of 95 Kg/mm ² or more at a high temperature of about 1200°C. The reason why a high-strength sintered body can be obtained according to the method of the present invention is considered to be as follows. That is, Si ₃ N ₄ with an α-type structure, which is the main raw material, is converted into a β-type structure during the high-temperature sintering process, and Si ₃ N ₄
The grains grow into long columnar shapes. On the other hand, aluminum nitride is compounded with silica to create a high melting point "Al-Si-O-N" compound, which absorbs and eliminates the undesirable silica and volatile components during sintering (Si _3-x Al _x ) N _4-x O
Prevents the transformation of _x・Re ₂ O ₃ into a low melting point compound or regression to glassy state. Therefore, a dense sintered body and a high quality sintered body free from the above-mentioned substances that contribute to a decrease in heat resistance can be obtained. The heat-resistant, high-strength sintered body mainly composed of Si ₃ N ₄ sintered body thus obtained contains only a high-melting point compound as a high-melting point grain boundary phase, and is composed of long columnar Si ₃ N ₄ sintered bodies. It is understood that the ease of crystal growth, sintering, and densification makes it possible to easily obtain a sintered body with good reproducibility and high strength even at high temperatures. Next, examples of the present invention will be described. Example 1 50 g of a mixed powder obtained by mixing Si ₃ N ₄ powder with a particle size of 1.2 μm and Y ₂ O ₃ powder with a particle size of 1.0 μm in an equimolar ratio containing 80% α-type structure was placed in an AlN crucible, and N ₂ gas Inside
Firing was performed at 1650°C for 4 hours. The thus obtained (synthesized) calcined powder was examined by X-ray diffraction.
It consisted of a single phase of Si ₃ N ₄・Y ₂ O ₃ . 7 parts by weight of the above Si ₃ N ₄ Y ₂ O ₃ powder and 85% α-type structure
and particle size 1.2 μm containing 1.2% silica component SiO ₂
91 parts by weight of Si ₃ N ₄ powder and additionally AlN with an average particle size of 1.3 μm
The raw material is a powder obtained by pulverizing and mixing 2 parts by weight of powder,
A molded body of 30 x 30 x 10 mm was obtained. This molded body is heated with _N2
Sintering was performed under pressure and heating in an atmosphere at 500 kg/cm ² at 1780° C. for 3 hours. As a result of measuring the density and bending strength of the sintered body thus obtained,
Density is 3.270g/cc, bending strength is 94.5kg/cc at room temperature
mm ² and 97.1 Kg/mm ² at 1200°C. Note that the bending strength is a value measured with 3-point bending and a span of 20 mm. The sintered bodies produced under the same conditions as above except that the molded bodies were 30×30×20 to 40 mm also exhibited approximately the same density and bending strength. Furthermore, the reliability of the strength of these sintered bodies was high, for example, the Weibull coefficient was 19 at room temperature. In contrast to the above example, a sintered body was manufactured under the same conditions except that the ratio of each raw material component was changed as another example, and the results of evaluating the characteristics are also shown in Table 1. The table also shows, as a comparative example, a characteristic example of a sintered body that does not contain AlN as a raw material component.

[Table] Example 2 Si ₃ N ₄ powder with particle size of 1.2 μ containing 85% α-type structure and particle size
Equimolar ratio mixed powder with 0.8μm _{Y2O3 powder} , particle size
Mixed powders were prepared by blending 0.3 μm Al ₂ O ₃ powder in the composition ratio shown in Table 2. These prepared powders were placed in an AlN crucible and placed in an _N2 atmosphere.
Firing was performed at 1600 to 1700° C. for 1 to 5 hours to synthesize a powdery compound having an X-ray structure of Si ₃ N ₄ .Y ₂ O ₃ type. 5 parts by weight of the Si ₃ N ₄ Y ₂ O ₃ type phase powder synthesized as above, 94 parts by weight of Si ₃ N ₄ powder containing 85% α-type structure and having a particle size of 1.2 μm, and further a particle size of 1.3 μm.
Using a mixed powder with 1 part by weight of AlN powder of m as raw material,
Molded bodies of 50 x 50 x 20 mm were made and sintered under pressure and heat under the same conditions as in Example 1 to obtain sintered bodies. The bending strength at 1200°C determined for each of the sintered bodies thus obtained is also shown in Table 2. Note that sample a in Table 2 is a comparative example.

[Table] Example 3 Si ₃ N ₄ powder with a particle size of 1.5 μm containing 96% α-type structure,
A mixed powder with an equimolar ratio of Ce ₂ O ₃ powder or Sm ₂ O ₃ powder with a particle size of 1.0 μm was calcined at 1650°C for 2 hours in an N ₂ atmosphere to form Si ₃ N ₄・Ce ₂ O ₃ or Si ₃ N _Four ·
Sm ₂ O ₃ was synthesized respectively. Next, these Si ₃ N ₄・Ce ₂ O ₃ , Si ₃ C ₄・Sm ₂ O ₃ ,
Si ₃ N ₄ powder (Si ₃ N ₄ ) powder (a) with a particle size of 1.0 μm containing 96% α-type structure, 1.1 μm particle size containing 85% α-type structure
Si ₃ N ₄ powder (Si ₃ N ₄ powder (b)) and particle size for each
1.3μ AlN powder was blended at the composition ratio (parts by weight) shown in Table 3 to prepare mixed powders.
Using these mixed powders, a molded body of 30 x 30 x 20 mm was first made, placed in a carbon mold, and heated at 1800 mm.
A sintered body was obtained by hot pressing at 450 Kg/cm ² at 3°C. The results of determining the density and bending strength of this sintered body are also shown in Table 3.

【table】

[Table] In each of the above Examples, cases where the a-group element was Y, Ce, or Sm were shown, but similar results were obtained when other elements such as La, Nd, Pr, or Dy were used. As is clear from the above examples, according to the method of the present invention, a heat-resistant sintered body (material) that maintains high mechanical strength even at high temperatures can be obtained. Thus, the method of the present invention can be said to be suitable for manufacturing large parts that require heat resistance and high strength at high temperatures, since desired sintered bodies can be easily obtained with good reproducibility.

Claims (1)

[Claims] 1. Silicon nitride powder 98.9 containing 60% or more α-type structure
~75% by weight, general formula, (Si _3-x Al _x )N _4-x O _x・Re ₂ O ₃ (In the formula, Re is an element from group a of the periodic table, x is 0 to 0.5
A composition containing 1 to 20% by weight of a powdery substance represented by the number of 1 to 20% by weight of aluminum nitride powder and 0.1 to 5% by weight of aluminum nitride powder was heated at 1700 to 1850°C under a pressure of at least 100 kg/ ^cm3.
A method for producing a heat-resistant, high-strength sintered body, which is characterized by pressurizing and sintering.